Typically, a wafer board panel comprises layers of wood flakes or wafers formed into a composite structure using a resinous binder. The preparation of wafer board panels is complex, but broadly consists of two principal stages. The first stage comprises the preparation of the wafers and admixing thereof with the binder to form a loose layer or mat. The second stage involves subsequent compression and heating of the mat to cure the binder and form the consolidated panel.
At present, wafer board is usually manufactured in the form of planar or flat sheets. Wafer board is a recognized structural panel, finding wide application in the construction industry, particularly as a plywood substitute in residential construction.
Improvement in performance characteristics of flat wafer board panels has been attained by optimization of such parameters as wafer orientation, wafer geometry, resin selection and content, and the like.
By wafer orientation is meant orientation through a degree of rotation which may range from the longitudinal to the transverse directions. Furthermore the orientation may take place in a layered or non-layered manner. Stated otherwise, the outer faces of the board only may contain orientated wafers whereas the core may contain wafers in random orientation. Alternatively, the orientation may be such that all the layers throughout the board are consistently aligned in one direction.
After exhaustive optimization studies of planar wafer board panels it was postulated that its flexural strength characteristics could be improved if a corrugated configuration was imparted thereto. The fundamental concept of corrugating materials to thereby improve the structural properties is not a novel one. Indeed, corrugated wafer board panels per se have previously been manufactured in the industry. However, the wafer board panels prepared by these prior art techniques do not have the desired structural strength properties because they do not have a substantially uniform density.
Until recently, wafer board panels were manufactured in the form of planar or flat sheets. However, as disclosed in U.S. Pat. No. 4,616,,991 and 4,625,138, the present applicant has developed an apparatus and process for the manufacture of panels having a wave-like or corrugated configuration. Such waveboard panels have improved structural strength properties, relative to planar panels.
This prior patented apparatus involves a pair of opposed, spaced-apart, upper and lower platens. Each platen is formed of adjacent lengths of chain-like links. When the lengths are pushed inwardly from the side, they shift from a planar to an undulating corrugated form.
The process steps involved:
distributing a mat of loose wood wafers between the upper and lower platen surfaces while they are maintained in the planar configuration; PA1 biasing the platens together to pre-compress the mat, and thereby substantially fixing the wafers together to limit their further relative movement; PA1 converting the two platen surfaces, still in pressing association with the mat, from the planar to the corrugated configuration; and PA1 then applying additional pressure and heat for a sufficient time to cure the binder and produce a corrugated waveboard panel.
The main advantage inherent in the patented process was that the panel product so formed was characterized in having a substantially uniform density. This was achieved because the wafers were fixed by the pre-compression step and because the mat was not significantly stretched or elongated during the conversion from the planar to the corrugated configuration.
It will be also noted that the waferboard panel product formed using the particular mechanical assembly described hereabove has a generally sinusoidal configuration. The peaks and troughs of the panel have a generally rounded profile. Furthermore, it is to be noted that the panel prepared using this apparatus is characterized in having a high amplitude. By high amplitude is meant a wave amplitude wherein the amplitude of the board centre line is greater than half the panel thickness. The amplitude of the board centre line is defined as the maximum distance between the neutral axis ( or line of symmetry) and the board centre line.
Low amplitude is defined herein as that wherein the wave amplitude is equal to, or less than one half of the thickness of the panel.
By thickness is meant the distance between the upper surface and the lower surface.
Whilst the above-described apparatus provides a useful product it will be readily appreciated by one skilled in the art that the cost of manufacture of such corrugating platens is significant.
In summary, therefore, prior to the development of the present invention, it had been appreciated that by having a high amplitude wave-board prepared using the above-described platen assembly a marked improvement in mechanical performance characteristics of the panel was obtained. However, what had not been realised was that providing even a minor degree, or very low amplitude, of corrugation to the board would result in markedly improved properties as compared to a planar panel.